PIN diodes are semiconductor structures having a p-type region, an n-type region, and an intrinsic region between the n-type and p-type regions. In light emitting diodes (LEDs) and laser diodes (LDs), electrons and holes injected from the p-type and n-type regions recombine within the intrinsic region, generating light. The type of materials used for the p-type, intrinsic, and n-type regions in the LED determine the wavelength of light emitted from the device. In particular, LEDs that emit light in the deep ultra-violet (deep-UV) wavelengths (<280 nm) often use group-III metal nitride semiconductor materials, such as aluminum nitride (AlN) or aluminum gallium nitride (AlGaN).
Several critical problems limit deep-UV LED performance with regards to light emission and thermal characteristics. Such limiting factors include current crowding, thermal management, and light extraction. Current crowding is characterized by a non-uniform distribution of current density through a semiconductor. Within a deep-UV LED or LD, current crowding can be caused by high defect densities in the grown group-III nitride films, high resistivities of the n-type and/or p-type regions, and/or low electron and hole carrier mobilities in the films.
Non-uniform distribution of current density contributes to non-uniform electron-hole recombination (EHR) in the light emitting region (e.g., the intrinsic region) of the LED. The regions with lower EHR rates produce less light than the regions with high EHR rates, causing less light emission from the LED in comparison to an ideal LED exhibiting uniformly high EHR rates. Further, current crowding contributes to the formation of “hot spots” (i.e., localized heating) in LEDs, which can result in thermal runaway in extreme cases.
Attempts to mitigate current crowding include attempts to reduce defect densities within the intrinsic region, attempts to improve conductivities within the p-type and/or n-type regions, attempts to improve the semiconductor device geometries (e.g., using about 10 micron width mesas with rectangular or hexagonal shapes), or to utilize different semiconductor device geometries (e.g., forming a vertical conduction device structure using a conducting substrate such as SiC).
In addition to current crowding, deep-UV LED performance is also often limited by inefficient light extraction. That is, group-III metal nitride materials have an inherently high refractive index, which limits the amount of the light generated within the LED that can escape from the surface. Efforts have been made in surface texturing to improve an escape cone of light from the surface. While such solutions have had some success in improving the light emission from deep-UV LEDs, they are still far from achieving optical power densities of commercial significance when compared to UV gas-discharge lamp technologies.